1. Field of the Invention
The present invention relates to methods and devices for monitoring alarm systems, and in particular to a method and apparatus for monitoring an alarm system for determining whether an alarm in the system has inadvertently become isolated from the environment it is intended to protect.
2. Description of the Prior Art
Danger alarm systems must function with a high degree of reliability requiring that non-functional alarm units within the alarm system must be quickly recognized and indicated as such. This is particularly true for automatic alarm systems such as fire alarm systems. In addition to disruptions which simulate alarm conditions and falsely cause the generation of an alarm signal, other types of alarm malfunctions are possible. One such type of malfunction is that an alarm may be correctly connected to an alarm line and emit a correct signal in its quiescent state, however, the evaluation circuitry associated therewith may malfunction so that upon the occurrence of a true alarm condition the quiescent signal emitted from the alarm unit is not changed into a alarm signal, and thus no alarm indication occurs at the central station.
Another possible malfunction is that an alarm unit may be properly connected to an alarm line, however the alarm unit may for some reason become isolated from its environment as a result of a high degree of contamination or as a result of an uncorrected structural change in the alarm unit itself. This may occur, for example, when painting is being done in the area monitored by an alarm unit, during which it is customary to cover items such as alarm units with a plastic or other covering material so as to avoid paint spattering thereon. Such a covering may inadvertently be left on the alarm unit thereby isolating the alarm unit from its environment so that the alarm unit cannot emit an alarm signal when alarm conditions occur.
Conventional alarm systems as described, for example, in U.S. Pat. No. 4,222,041, generally include a special monitoring circuit for monitoring the individual components of the alarm system and emitting a fault or malfunction signal to the central station when a particular malfunction is detected. A theoretically complete monitoring system which has the ability to detect each of the large variety of malfunction possibilities is generally not utilized because of the considerable equipment outlay necessary to adequately monitor all possible faults. Conventional alarm systems, for this reason, therefore generally require periodic visual inspections of the alarm units in order to determine a malfunction such as the type described above wherein the alarm unit is physically isolated from its environment. Depending upon the frequency of such visual inspections, an alarm unit may remain isolated from its environment for some time before the problem is discovered, during which time the alarm unit cannot function to monitor its assigned area for danger conditions.